1. Field of the Invention
The present invention relates to a method of generating a toric surface on a molding tool for molding contact lens molds. Additionally, the invention relates to a collet assembly which may be employed in the method.
2. Description of the Related Art
Contact lenses having a toric optical zone (commonly referred to as "toric contact lenses") can be used to correct refractive abnormalities of the eye associated with astigmatism, wherein the toric optical zone provides cylindrical correction for the astigmatism. Since astigmatism is often associated with another refractive abnormality, such as myopia (nearsightedness) or hypermetropia (farsightedness), toric contact lenses are commonly prescribed with a negative or positive spherical correction to correct myopic astigmatism or hypermetropic astigmatism. The optical requirements of a toric contact lens are well known in the art, as discussed in U.S. Pat. No. 4,680,998 (Council, Jr.), the disclosure of which is incorporated herein by reference, and various methods are known for producing toric contact lenses.
U.S. Pat. No. 3,079,737 (Kratt et al.) relates to a toric lens tool for holding a lens blank on which a toric surface is machined. A toric surface is generated on the lens blank by a method comprising: warping (or "crimping") the lens blank by imparting diametrically opposed forces to peripheral portions of the lens blank; lathe cutting a spherical base curve in the convex posterior surface of the warped lens blank and polishing the base curve while the blank remains oblately distorted; and releasing the lens blank from the tool, whereby the blank returns to its original shape and the base curve assumes a toric shape.
This crimping method requires individually crimping and lathe cutting each lens blank to generate the desired toric surface, which is time consuming and labor intensive especially for the production of large quantities of lenses. Further, the reproducibility and consistency of the toric surface is entirely dependent on the individual crimping operation.
U.S. Pat. No. 5,110,278 (Tait et al.) relates to a method of injection molding a toric lens casting mold which includes a hollow, cup-like top portion having a toric surface. Lens buttons having a precision toric base curve and a generally flat front surface are molded in the cup-like top portion of the mold arbor. The toric surface of the cup-like top portion is formed upon injection molding from a toric core pin which has a concave toric end surface. Subsequent to molding the button with the toric lens surface, the other lens surface is lathe cut from the generally flat front surface. The patent discloses that the toric surface on the toric core pin may be produced by known manufacturing procedures, including lathe machining followed by polishing, electroforming, or electro discharge machining followed by polishing.